Zemmels review: Make text descriptions of HUCs clearer

I'd recommend adding an expansion/definition of the "HUC" acronym to the Area-Based Runoff section. I see there's a discussion of HUCs about halfway through that first paragraph. Perhaps that could be moved up to before/around the acronym's first usage? Great point. We used this as an opportunity to expand the glossary to include a definition for hydrologic units, complete with information on hydrologic unit codes and how they're delineated. We then link to the glossary term in the Calculations Quick Reference document. It also prompted us to clean up the glossary a bit and get it into alphabetical order!